The Planning Use class D1 would include the building being used as a place of worship as one of the types of use within that Use Class. If the current use is D1, it is likely that planning permission would be required to convert the building to flats. That decision would be made by your local Planning authority which would be Harrow Council in this case. Some other buildings with a different range of uses can be changed to residential without the need for planning permission but subject to a number of specific criteria. This procedure is known as Prior Approval and would normally include matters such as potential flooding, contamination, highways and transport issues and impacts of noise from commercial premises on the intended occupiers of the development. The Council will be required to respond to the pre-planning application which presumably seeks to determine whether a full planning application is needed or whether the Prior Approval would be the appropriate legal route.